Method of zeolite synthesis with homogeneous shaking out of the medium, device and application

ABSTRACT

A method is provided for zeolite synthesis from a synthesis medium containing in particular a trivalent aluminum source, a tetravalent silicon source, at least an alkaline or alkaline-earth cation in hydroxide form and water in a reactor containing a solid helical moving body in a guiding tube defining an internal space and a space external to the tube. The invention is characterized in that the synthesis medium is circulated in the reactor in a continuous flow passing through the internal space then the space external to the tube and returning to the internal space, only driven by the roation of the solid helical moving body, at a speed of less than 500 rpm, in the tube which is kept fixed, the synthesis medium being kept at a temperature suitable for at least one of the ripening operations or the crystal growth of the synthesized zeolite. The invention also is directed to a device for implementing the method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for zeolite synthesis and toa device for implementing it. The present invention relates moreparticularly to the synthesis of zeolites of all kinds in a stirredmedium with a view to improving the manner of formation of the zeolitecrystals and their quality.

2. Background of the Art

To obtain zeolites, it is usual to form so-called crystallizingsolutions or gels containing a source of trivalent aluminium, a sourceof tetravalent silicon, at least one alkali or alkaline-earth metalcation in hydroxide form, water and optionally an organic structuringagent. However, from these constituents it is possible to obtain quitedifferent zeolites, such as offretite (OFF), mazzite (MAZ) or zeolite L(LTL) (S. Ernst and J. Weitkamp; Catalysis Today 19, 1994, 27-60) byvarying their distribution or the subsequent hydrothermal treatmentneeded for the crystallization, or else the stirring of the medium.

Other syntheses are obtained from gels into which natural or syntheticcrystalline aluminosilicates or even crystals of zeolites are introducedas replacement for amorphous sources of aluminium. Such substitutionshave made it possible to obtain a homogeneous growth of the crystals oftarget zeolites, regardless of whether this is done in an unstirredmedium or a stirred medium, because of the slow and uniform dissolutionof these aluminosilicates or of these crystals of source zeolite in thesynthesis medium. It has thus been possible to synthesize zeolites ofmazzite type (Dwyer, U.S. Pat. No. 4,091,007, Fajula, U.S. Pat. No.4,891,200, Cannan, U.S. Pat. No. 4,840,779 and Di Renzo, FR 2,651,221).

The prior art is rich in examples which show that small variations inthe composition of the synthesis gel or in the crystallizationconditions can result in the synthesis of zeolites which are verydifferent in kind or even of amorphous compounds. Another factor whichaffects the growth of the crystals is the stirring (or the absence ofstirring) of the synthesis medium, independently of the composition ofthe crystallizing solution or gel. Thus, in order to obtain a largenumber of zeolites, a person skilled in the art advises strongly againstthe use of stirrers during the crystallization of the zeolite, bothduring the maturing stage and during the crystal nucleation stage. Ithas been ascertained that, depending on the composition of thecrystallizing solution or gel, it is possible to see appearing either amixture of faujasite and of gmelinite in a static medium, or of zeoliteP in a medium stirred with an anchor stirrer (D. E. W. Vaughan, Chem.Engin. Progress, 84(2), 1988, pp 25-31). In fact, the high shear rategenerated by the stirrer localized only in a portion of the synthesismedium considerably modifies the process of germination and of growth ofthe crystals of zeolites and their nature, even if the initialcomposition of the gel is the same. In parallel, in the case of zeoliteswhich are usually prepared in a stirred medium, like, for example,faujasite, a lowering has been noted in the crystallinity of thefaujasite obtained when the stirrer speed and hence the local shear areincreased (R. M. Barrer, Hydrothermal Chemistry of Zeolites, AcademicPress, 1982, p. 171).

It is for the abovementioned reasons that many syntheses of zeolites arecarried out statically, that is to say without stirring, because thispromotes the slow growth of the crystals and the appearance andstabilization of zeolite phases which are thermodynamically not verystable. Moreover, it has not yet been possible to reproducesynthetically many natural zeolites in the pure state or as by-productsof synthesis of other zeolites, regardless of whether this is in astirred medium or a static medium.

Furthermore, it is well known to the manufacturers of zeolites that thetransposition of some syntheses to an industrial scale is difficult oreven impossible. It often results in low zeolite yields and in degreesof crystallinity which are much lower than those generally obtained onthe laboratory scale (D. W. Breck, Zeolite Molecular Sieves, John Wileyand Sons, 1974, p. 725-731).

For the above reasons it is impossible to synthesize zeolites byprocesses that combine two types of stirring including anchor stirringand stirring by means of a propeller placed in a cylinder in perpetualmotion, as proposed in patent U.S. Pat. No. 1,115,791, since thesestirring actions induce strong variations in shear at the anchor and theedges of the cylinder in motion, which do not promote the maturing andthe stabilization of the crystal phases.

The present invention is therefore aimed at a process for zeolitesynthesis which makes it possible to stabilize more easily some zeolitephases that are highly unstable, even in static operation, to improvethe quantitative yields of synthesis in relation to those obtained in astatic or conventional stirred medium while shortening the synthesistime by virtue of better control of the transfers of matter and of heatin the synthesis gel or solution, and thus greatly to increase thedegree of crystallinity of the zeolites. In addition, the process aimedat can be easily transposed to an industrial scale.

SUMMARY OF THE INVENTION

The subject matter of the present invention is therefore a process forthe synthesis of a zeolite from a synthesis medium containing especiallya source of trivalent aluminium, a source of tetravalent silicon, atleast one alkali or alkaline-earth metal cation in hydroxide form andwater, performed in a reactor containing a solid helical rotor in aguide tube defining a space which is internal and a space which isexternal to the said tube, characterized in that the said synthesismedium is circulated in the reactor as a continuous flow moving throughthe space which is internal and then the space which is external to thesaid tube and returning to the internal space, solely by the rotation ofthe solid helical rotor, at a speed lower than 500 rev/min, in the saidtube held stationary, the synthesis medium being maintained at atemperature for carrying out at least one of the operations of maturingor of crystal growth of the zeolite to be synthesized.

The circulation of the said synthesis medium is ensured by thecontinuous or noncontinuous driving in rotation of the solid helicalrotor. It is obvious that it is also possible to cause the medium tocirculate by means of a number of solid helical rotors, each beingdriven in rotation in a guide tube held stationary in the reactor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The dimensional characteristics and the speed of rotation of the solidrotor are adapted to the synthesis medium, especially to its viscosity,in order to establish a nonturbulent flow regime in the reactor, that isto say a laminar or intermediate flow regime corresponding to a stirringReynolds number lower than 25,000 (see the collection entitled Mixing byNagata, Wiley, 1975). The speed of rotation of the helical rotor of 500revolutions/min corresponds to the maximum shear rate tolerated in orderto obtain a laminar or intermediate flow regime in gels for thesynthesis of zeolites.

In the course of the syntheses of zeolites in static medium, seedsappear randomly in the synthesis medium, that is to say in places whichare unpredictable and at various times, and this produces crystal sizedistributions which are broad and not very reproducible and results inthe presence of a number of intermediate crystalline or amorphousphases. In a conventional stirred medium the medium has a shear ratewhich is variable and often high in the region of stirring and virtuallynil in comparison outside the stirrer zone, and this interferes withhomogeneous germination. With the present invention the medium is madehomogeneous by the controlled stirring of the medium, that is to say bysetting all of the medium in motion in the guide tube(s) and outsidethereof. A consequence of this homogeneous flow is that shear rates,temperatures and compositions are obtained which are virtually identicalat all points, resulting in an equal chance of obtaining an identicalgermination process at the same time, which cannot be obtained usingconventional stirring processes or even in a static medium.

Also, besides the use of a solid helical rotor in order to make thesynthesis medium circulate without perturbing the flow regime and hencewithout appreciably varying its shear rate, the guide tube includes atleast one orifice in its lower part the shape of which has appreciablyonly a small influence on the shearing of the gel. An orifice is hereintended to mean the space between the lower end of the guide tube andthe bottom of the reactor or a plurality of holes cut out in the lowerend of the guide tube resting in the bottom of the reactor. It is easyfor a person skilled in the art to give any desired shape to theseholes, as their edges must not promote the nucleation of undesirablezeolite seeds, that is to say a heterogeneous nucleation.

Within the scope of the present invention the axis of at least onehelical rotor coincides with the axis of the reactor and that of theguide tube which contains it.

In a preferred embodiment of the invention the helical rotor isinscribed within a cylinder of revolution such that the ratio of thediameter of this said cylinder of revolution to the diameter of theguide tube which contains it varies from 0.4 to 0.99, and such that theratio of the diameter of this cylinder of revolution to the diameter ofthe reactor varies from 0.3 to 0.9.

In addition, in the reactor, the height of the guide tubes is chosensuch that it is smaller than that of the reactor, but preferably suchthat the ratio of the height of the said guide tube to the height towhich the reactor is filled by the synthesis gel or solution is smallerthan or equal to 0.95.

To implement the process according to the invention, the solid helicalrotor necessary for circulating the synthesis medium in the reactor willbe chosen from the group consisting of an Archimedes' screw, twistedbands or any other solid rotor which has at its periphery at least oneexternal ridge exhibiting a helical shape over the whole of its length,capable of establishing a nonturbulent flow regime, by driving inrotation. The preferred rotor of the invention is an Archimedes' screwdriven in a rotary motion by means of a motor.

The use of an Archimedes' screw in the case of which there is no pitchlimit makes it possible to ensure not only a homogeneous flow in thereactor at virtually constant shear, but also permits an immediatetransposition of the process of the invention to any scale and chieflyto an industrial scale.

In order to make use of the process of the invention, the ratio of thespeeds of the flows in opposite directions inside and outside each guidetube varies between 0.5 and 2.

In a particular embodiment of the invention the synthesis medium passesthrough a succession of reactors arranged in series or in parallel andconnected together, the gel recirculating in each reactor before beingdirected towards the next reactor. With this particular embodiment thestages of nucleation and of growth or of maturing of the gel, and itscrystallization, can be separated.

Another subject matter of the present invention is a device for makinguse of the process of the invention, shown in FIG. 1, which ischaracterized in that it consists of at least one open or closed reactor(1) containing a cylindrical guide tube (5), stationary in relation tothe reactor, with axis parallel to the walls of the reactor, comprisingat least one orifice in its lower part, this guide tube containing asolid helical rotor (6) inscribed within a cylinder of revolution ofdiameter varying from 0.3 to 0.9 times the diameter of the reactor andfrom 0.4 to 0.99 times the diameter of the guide tube, this said rotorbeing driven in rotation by a motor (8), external to the reactor, thesaid reactor including conduits (4) for injection of the compoundsforming part of the constitution of the gel, and optionally a device fordraining (2).

In a preferred form of the invention the guide tube containing thehelical rotor is placed on the axis of the reactor. It would notconstitute a departure from the scope of the invention if severalparallel guide tubes were installed, each containing a solid helicalrotor, one of which may be placed on the axis of the reactor.

In a reactor of the device of the invention the guide tube does notoccupy the whole height of the reactor; however, the ratio of the heightof the guide tube to the height of filling of the reactor is preferablykept lower than 0.95.

Among the solid rotors applicable to the present invention there are tobe found rotors of the group consisting of the Archimedes' screw,helical bands and any solid rotor which has at its periphery at leastone external ridge exhibiting a helical shape over the whole of itslength. The preferred rotor is an Archimedes' screw which is driven inrotation by means of a motor placed on its axis, its speed of rotationbeing preferably lower than 500 revolutions per minute. Any screwpitches are allowed for the Archimedes' screws within the scope of theinvention.

The device of the invention may consist of a single open or closedreactor of autoclave type including at least one guide tube containing ahelical rotor and capable of operating under pressure or open to theatmosphere.

In another embodiment of the device the latter may consist of aplurality of reactors placed in series, each containing a single guidetube equipped with an Archimedes' screw connected to a motor andoptionally including at its base a device for shutting/draining thereactor. The Archimedes' screws may all be identical or different, mayrotate at the same speed or otherwise and may be coupled to the samemotor or otherwise.

These devices according to the invention facilitate industrialextrapolation, but they also make it possible to work at lowercrystallization temperatures permitting the synthesis of zeolites atatmospheric pressure, in contrast to the known prior art.

The present invention also relates to the application of the process andof the device according to the invention to the synthesis in a mediumstirred at constant shear of zeolites such as EMO, EMT, gmelinite andmazzite.

The understanding of the process and of the device of the invention willbecome clearer in the light of the figures described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in section, a device for noncontinuous zeolite synthesis.

FIG. 2 shows, in section, a second device for noncontinuous synthesis.

FIG. 3 shows the section of a device for continuous synthesis ofzeolite.

In FIG. 1 the reactor (1) is shown in the form of an autoclave which canoperate at atmospheric pressure or under pressure, including acylindrical body (1a) equipped with a device for draining (2) and a lid(1b). This lid (1b) is pierced with an orifice (3) and with conduits (4)for delivery of the constituents forming part of the composition of thesynthesis solutions or gels. A guide tube of cylindrical shape (5) isfixed by any means inside the body of the reactor (1a); its axiscoincides with that of the reactor (1). It contains an Archimedes' screw(6) whose axis of revolution coincides with the above two axes. Theextension (7) of the said screw (6) passes through the lid (1b) throughthe orifice (3) and is connected to a motor (8) which can turn the saidscrew (6) about itself.

In FIG. 2 a reactor (1) is placed, including, as in FIG. 1, a lid (1b)but this time pierced with a number of orifices (3) (three are shown)and including a number of conduits for delivery of the components of thesynthesis gel, and a body (1a) including a device for draining (2). Thereactor contains at least three guide tubes (5a, 5b and 5c), eachcontaining an Archimedes' screw (6a, 6b and 6c), the axis of the screw(6b) coinciding with those of the guide tube (5b) and of the reactorbody (1a). The three screws (6a, 6b and 6c) are joined to three motors(8a, 8b and 8c) by the extensions (7a, 7b and 7c) passing through theorifices (3) in the lid (1b).

FIG. 3 shows a series of reactors (10) placed in series and connected toone another, exhibiting at the base the form of a venturi (11) theorifice of which is equipped with a shutting/draining device (12)capable of shutting the said venturi (11) and placed at the bottom ofthe reactor (10). Each reactor (10) is equipped with a guide tube (13),each containing an Archimedes' screw (14). All these screws (14) ofidentical or different pitches, are either connected together andcoupled to the same motor (15) placed at the head of the first reactor(10a), or connected to different motors which are not shown in thefigure, enabling these screws (14) to be driven in rotation but atdifferent speeds. The first reactor (10a) has conduits (16) for deliveryof the necessary components of the gel. Such conduits could also bepresent at the heads of the other reactors (10b), . . . (10i). At thebottom of the reactor (10i) a conduit (17) makes it possible to recoverthe crystallized gel which is entrained towards the units for separationand subsequent treatment of the crystals. The partial or completeclosure of the shutting/draining devices (12) makes it possible toprovide partial or complete recirculation of the synthesis gels in eachreactor (10).

Examples are given below, without any limitation being implied, to showthe effectiveness of the process and of the device according to theinvention. All the zeolites prepared hereinafter are designated by theirreference name, published in the "Atlas of Zeolite Structure Types",third revised edition, Butterworth and Heinemann, London, 1992.

EXAMPLE I

This example aims to show the superiority of the process of theinvention and of the corresponding device for the preparation of EMO.

For this purpose several samples are prepared of the same gel forsynthesis of zeolite of faujasite structure of Si/Al ratio >3.5 in thepresence of crown ether "15-Crown-5" or 15-C-5, on the one hand usingthe conventional process in static medium in an autoclave and, on theother hand, in a stirred medium either according to the invention or bymeans of a conventional anchor-type stirrer. The gels thus prepared areof stoichiometry identical with:

10 SiO₂, Al₂ O₃, 2.1 Na₂ O, 0.4 15-C-5, 100 H₂ O.

The operation is carried out by introducing successively into a reactorof 2 1 capacity, in the case of each of the samples, at:

t=0, 831.6 g of demineralized water

t=15 min, 84.4 g of 15-C-5

t=45 min, 77.5 g of NaOH

t=75 min, 170.8 g of NaAlO₂

t=105 min, 1406.3 g of Ludox AS 40 colloidal silica marketed by DuPontde Nemours.

The reactor temperature is maintained at at least 100° C. throughout theperiod of formation of the gel and then for some time longer for theformation of the EMO crystals.

Table I below shows the crystallinity characteristics of the EMOcrystals obtained, as a function of the stirring of the synthesismedium, of the gel temperature and of the gel maturing period. Withinthe scope of the process according to the invention an Archimedes' screwis employed, of pitch equal to 81 mm, of diameter equal to 81 mm, in acylinder of 89 mm diameter, in an autoclave of diameter equal to 150 mm.The screw is driven in rotation at 250 revolutions/min.

                  TABLE I                                                         ______________________________________                                                                     crystal-                                                         v            lization       crys-                                   type of   (rev/  τ T    t     crystal-                                                                            tal                             Sample                                                                              synthesis min)   (s.sup.-1)                                                                          (° C.)                                                                      (days)                                                                              linity                                                                              size                            ______________________________________                                        EMO 1 static    --     0     110  12    100   1-2                             EMO 2 static    --     0     110  5     amor-                                                                         phous                                 EMO 3 static    --     0     100  5     amor-                                                                         phous                                 EMO 4 stirred/  200    het.  110  5     amor-                                       anchor           67               phous                                 EMO 5 stirred/  250    homo. 100  5     100   0.5                                   Archimedes'      54                                                           screw                                                                   EMO 6 stirred/  250    homo.  90  11    130   0.5                                   Archimedes'      54                                                           screw                                                                   ______________________________________                                         *v = speed of rotation of the stirrer.                                        *τ = shear rate of the stirrer.                                           *het. = shear rate very different near the anchor and in the remainder of     the solution.                                                                 *homo. = shear rate gradient close to 0.                                      *crystallinity measured by Xray diffraction.                             

The three faujasites (FAU) obtained, EMO 1, EMO 5 and EMO 6 areidentical in chemical composition.

It is found, according to this table, that in a static regime acrystallization temperature of 110° C. and a crystallization period of12 days are needed to obtain EMO in which the crystal size is from 1 to2 μm (EMO 1), whereas only 5 days are needed for crystallizing accordingto the process of the invention at 100° C. EMO in the form of smallcrystals (0.5 μm--EMO 5). The other tests reported show that it isimpossible to crystallize EMO statically in less than 12 days or at atemperature of 100° C., even in a medium stirred by means of ananchor-type stirrer driven in rotation at 200 revolutions/min and at ahigh shear rate (EMO 2, EMO 3 and EMO 4).

This example therefore emphasizes that the synthesis in a stirred mediumaccording to the invention results in shorter crystallization periods(EMO 5) with at least the same, if not better, quality of crystals ofFAU obtained (crystal size), which makes it possible to lower thecrystallization temperature (EMO 6) and to work at atmospheric pressure.

EXAMPLE II

This example aims to show the superiority of the process of theinvention and of the corresponding device for the preparation of EMT.

For this purpose several samples are prepared of the same gel forsynthesis of zeolite of faujasite structure of Si/Al ratio >3.5 in thepresence of crown ether "18-Crown-6" or 18-C-6, on the one hand by theconventional process in a static medium in an autoclave and, on theother hand, in a medium stirred either according to the invention or bymeans of a conventional anchor-type stirrer. The gels thus prepared areof identical stoichiometry:

10 SiO₂, Al₂ O₃₁ 2.1 Na₂ O, 0.4 18-C-6, 100 H₂ O.

The operation is carried out by introducing successively into a reactorof 2 l capacity, for each of the samples, at:

t=0, 831.6 g of demineralized water

t=15 min, 84.4 g of 18-C-6

t=45 min, 77.5 g of NaOH

t=75 min, 170.8 g of NaAlO₂

t=105 min, 1406.3 g of Ludox AS 40 colloidal silica marketed by DuPontde Nemours.

The reactor temperature is maintained at at least 100° C. throughout theperiod of formation of the gel and then for some time longer for theformation of the EMT crystals.

Table II below shows the crystallinity characteristics of the EMTcrystals obtained, as a function of the stirring of the synthesismedium, of the gel temperature and of the gel maturing period. In thecase of the samples according to the invention the operation is carriedout in a device identical with that described in Example I in identicaloperating conditions.

                  TABLE II                                                        ______________________________________                                                                     crystal-                                                         v            lization       crys-                                   type of   (rev/  τ T    t     crystal-                                                                            tal                             Sample                                                                              synthesis min)   (s.sup.-1)                                                                          (° C.)                                                                      (days)                                                                              linity                                                                              size                            ______________________________________                                        EMT 1 static    --     0     110  12    100   1-2                             EMT 2 static    --     0     110  5     amor-                                                                         phous                                 EMT 3 static    --     0     100  5     amor-                                                                         phous                                 EMT 4 stirred/  200    het.  110  5     amor-                                       anchor           67               phous                                 EMT 5 stirred/  250    homo. 100  7     100   0.5                                   Archimedes'      54                                                           screw                                                                   ______________________________________                                         *v = speed of rotation of the stirrer.                                        *τ = shear rate of the stirrer.                                           *het. = shear rate very different near the anchor and in the remainder of     the solution.                                                                 *homo. = shear rate gradient close to 0.                                      *crystallinity measured by Xray diffraction.                             

The two faujasites (FAU) obtained, EMT 1 and EMT 5 are identical inchemical composition.

It is found, according to this table, that, as in the case of Example I,the crystallization takes place in conditions which are more favourablethan those employed by a person skilled in the art, that an at leastidentical, if not better, EMT crystallinity is obtained and that it ispossible to work more quickly at lower temperatures, even at atmosphericpressure.

EXAMPLE III

This example aims to show the superiority of the process of theinvention and of the corresponding device for the preparation ofmazzite.

Several samples are prepared of the same synthesis gel of followingstoichiometry:

5.3 Na₂ O, Al₂ O₃, 0.3 TMA₂ O, 15 SiO₂, 270 H₂ O

The various gel samples are prepared with stirring which is eithermechanical, at approximately 150 revolutions/minute by means of ananchor-type stirrer, or according to the process according to theinvention by means of an Archimedes' screw of pitch equal to 81 mm, ofdiameter equal to 81 mm in a guide tube of 89 mm diameter, rotating at aspeed of 250 rev/min. The samples are kept at ambient temperaturethroughout the period of introduction of the components of the gel.

Thus, for each sample, 1902.3 g of demineralized water in which 150.6 gof sodium hydroxide and then 26.2 g of tetramethylammonium TMAC1 aredissolved gradually are introduced into a reactor of 2 1 capacity. Tothis solution are gradually added 333.9 g of Zeosil 1165 MP marketed byRhone Poulenc, forming the source of silica of the zeolite, and thenstirring is maintained for one hour. The source of aluminium, in thiscase consisting of 130.9 g of siliporite NaX marketed by CECA is addednext and then the stirring is maintained for two hours. The temperatureof the gel is next raised to 115° C. in the case of MAZ 1 and MAZ 2 orto 100° C. in the case of MAZ 3, and then maintained at these values.

According to the conventional synthesis the gel is stirred with ananchor stirrer rotating at 150 rev/min, at autogenous pressure, untilthe mazzite crystallizes. According to the process of the invention thegel is stirred at atmospheric pressure.

The conditions of synthesis and of crystallization and the crystallinityof the zeolites obtained for each sample are collated in Table IIIbelow.

                  TABLE III                                                       ______________________________________                                                                     crystal-                                                         v            lization                                               type of   (rev/  τ T     t     crystallinity                        Sample                                                                              synthesis min)   (s.sup.-1)                                                                          (° C.)                                                                       (days)                                                                              (%)                                  ______________________________________                                        MAZ 1 stirred/  150    het.  115   40    100 MAZ                                    anchor           50                                                     MAZ 2 stirred/  15.0   het.  115   26    amorphous                                  anchor           50                + MAZ                                MAZ 3 stirred/  250    homo. 100   26    100 MAZ                                    Archimedes'      54                                                           screw                                                                   ______________________________________                                         *v = speed of rotation of the stirrer.                                        *τ = shear rate of the stirrer.                                           *het. = shear rate very different near the anchor and in the remainder of     the solution.                                                                 *homo. = shear rate gradient close to 0.                                      *crystallinity measured by Xray diffraction.                             

It is found, according to this table, that the synthesis of mazzite with100% crystallinity in anchor-type stirred medium is less fast than thatcarried out according to the process of the invention at a temperaturewhich is 15° C. lower.

EXAMPLE IV

This example is aimed at showing the superiority of the process of theinvention and of the corresponding device for the preparation ofgmelinite, compared with the traditional processes of synthesis in astatic regime or in a medium stirred at a high shear rate.

Several samples are prepared of the same synthesis gel obtained byproceeding as follows:

Into a reactor 2 liters in capacity are introduced, at,

t=0, 1164.4 g of demineralized water,

t=15 min, 136 g of PEO (polyethylene oxide of average molecular weightof 3400 g/mol), marketed by Aldrich,

t=45 min, 77.5 g of sodium hydroxide,

t=75 min, 170.8 g of NaAlO₂,

t=105 min, 1406.8 g of Ludox AS 40 colloidal silica marketed by DuPontde Nemours.

The gel thus prepared has the following stoichiometry:

10 SiO₂, Al₂ O₃, 2.1 Na₂ O, 0.04 PEO, 140 H₂ O.

The results of the syntheses are given in Table V below.

                  TABLE V                                                         ______________________________________                                                                     crystal-                                                         v            lization                                               type of   (rev/  τ       t     crystal                              Sample                                                                              synthesis min)   (s.sup.-1)                                                                          T (° C.)                                                                     (days)                                                                              phases                               ______________________________________                                        GME 1 static    --     0     100   6     amorphous                                                                     + FAU                                GME 2 static    --     0     100   12    FAU +                                                                         εGME                         GME 3 stirred/  150    homo. 100   6     100 GME                                    Archimedes'      1950                                                         screw                                                                   GME 4 stirred/  350    homo. 100   5     100 GME                                    Archimedes'      4650                                                         screw                                                                   GME 5 stirred/  180    het.  100   5     amorphous                                  anchor           3600              + FAU                                GME 6 stirred/  180    het.  100   12    FAU + GME                                  anchor           3600                                                   ______________________________________                                         *v = speed of rotation of the stirrer.                                        *τ = shear rate of the stirrer.                                           *het. = shear rate very different near the anchor and in the remainder of     the solution.                                                                 *homo. = shear rate gradient close to 0.                                 

It is found that the synthesis in a static medium, regardless of thecrystallization period (up to 12 days) does not result in a puregmelinite phase (GME). The synthesis according to the invention, indifferent shear conditions (150 and 350 revolutions/min) results in anycase in a pure gmelinite phase, whereas the synthesis in an anchor-typeconventional stirred medium produces a mixture of gmelinite and offaujasite (predominant).

Furthermore, it was found that the pure GME obtained had an ovoidmorphology with a crystal size larger than 1 μm.

What is claimed is:
 1. A process for the synthesis of a zeolite from asynthesis medium containing a source of trivalent aluminum, a source oftetravalent silicon, at least one alkali or alkaline-earth metal cationin hydroxide form and water, comprising maintaining said synthesismedium at a maturing temperature or crystallizing temperature in areactor containing a solid helical rotor in a guide tube defining aspace which is internal and a space which is external to the said tube,wherein said synthesis medium is circulated in the reactor as acontinuous flow moving through the space which is internal and then thespace which is external to said tube and returning to the internalspace, solely by the rotation of the solid helical rotor, at a speedlower than 500 rev/min, in said guide tube held stationary.
 2. Theprocess according to claim 1, wherein the circulation of said synthesismedium is ensured by the continuous or noncontinuous driving in rotationof the helical rotor.
 3. The process according to claim 1, wherein thecirculation of the medium corresponds to a nonturbulent flow regime. 4.The process according to claim 1, wherein the guide tube is providedwith at least one orifice in its lower part allowing the synthesismedium to pass freely without perturbing the flow regime.
 5. The processaccording to claim 1, wherein the axis of the rotor coincides with theaxis of the guide tube which contains it and the axis of the reactor. 6.The process according to claim 1, wherein the helical rotor is inscribedwithin a cylinder of revolution such that the ratio of the diameter ofthe cylinder of revolution to the diameter of the guide tube varies from0.4 to 0.99.
 7. The process according to claim 1, wherein the helicalrotor is inscribed within a cylinder of revolution such that the ratioof the diameter of the cylinder of revolution to the diameter of thereactor varies from 0.3 to 0.9.
 8. The process according to claim 1,wherein the ratio of the height of the guide tube to the height of thereactor is [kept] smaller than equal to 0.95.
 9. The process accordingto claim 1, wherein the helical rotor is selected from the groupconsisting of an Archimedes' screw, a twisted band or any other solidrotor which has at its periphery at least one external ridge exhibitinga helical shape over the whole of its length.
 10. The process accordingto claim 9, wherein the helical rotor is an Archimedes' screw.
 11. Theprocess according to claim 10, wherein the ratio of the speeds of theflows in opposite directions inside and outside the guide tube variesbetween 0.5 and 2 in the reactor.
 12. The process according to claim 11,which is carried out in a plurality of reactors placed in series or inparallel, connected together, the gel recirculating in each reactorbefore being directed to the next reactor.
 13. A device for thesynthesis of a zeolite of the process defined by claim 1, which consistsof at least one open or closed reactor containing a cylindrical guidetube, stationary in relation to the reactor, with axis parallel to thewalls of the reactor, comprising at least one orifice in its lower part,the guide tube containing a solid helical rotor inscribed within acylinder of revolution of diameter varying from 0.3 to 0.9 times thediameter of the reactor and from 0.4 to 0.99 times the diameter of theguide tube, said rotor being driven in rotation by a motor, external tothe reactor, said reactor additionally including conduits for injectionof the compounds forming part of the reaction medium and optionally adevice for draining.
 14. The device according to claim 13, wherein theguide tube containing the helical rotor is placed on the axis of thereactor.
 15. The device according to claim 13, wherein the ratio of theheight of the guide tube to the height of filling of the reactor islower than 0.95.
 16. The device according to claim 13, wherein thehelical rotor are selected from the group consisting of an Archimedes'screw, helical bands or any other solid rotor which has at its peripheryat least one external ridge exhibiting a helical shape over the whole ofits length.
 17. The device according to claim 13, wherein the helicalrotor is an Archimedes' screw which is driven in rotation by means of amotor placed on the axis of the Archimedes' screw, the speed of rotationof the Archimedes' screw being lower than 500 rev/min.
 18. The deviceaccording to claim 13, wherein the device consists of a single autoclavereactor capable of operating under pressure or open to the atmosphere.19. The device according to claim 13, wherein the device consists of aplurality of reactors placed in series, each containing a single guidetube equipped with an Archimedes' screw connected to a motor andoptionally including at its base a device for shutting/draining thereactor, the Archimedes' screws being all identical or different,coupled to the same motor or otherwise rotating at the same speed orotherwise.